Understanding Sensor Technologies: Basics and Applications

1. Tutorial 5 Derek Wright Wednesday, February 16th, 2005

2. Sensors and Image Systems • Physical Principles of Sensors • Optical Imaging Systems • IR Imaging Arrays • Electronic Nose • Tactile Sensors and Arrays

3. Sensor Basics • Sensors are transducers • Transducers convert one form of energy to another • Alternator in your car turns mechanical into electrical • Engine converts chemical to thermal to mechanical • Eyes convert light into electrical

4. Sensor Basics • Sensors either • Directly convert one form to another • Use one form to change (modulate) another • Direct Conversion: • Solar panel: Light  Electricity • Thermocouples: Heat  Electricity • Modulating: • Thermoresistive, Optoresistive: Changing resistance must be have current driven through it to measure

5. Biological Sensor Arrays - Eyes • The eye is a biological form of a sensor array • It consists of an array of transducers (rods and cones) • The signals are transmitted by neurons along axons

6. Optical Imaging Systems • Array structures allow multidimensional measurement to occur • Optical Imaging Systems: • Charge Coupled Devices (CCDs) • CMOS Cameras • X-ray Imagers

7. Charge Coupled Devices • Incident photons cause creation of electron-hole pairs • Electrons move to insulator boundary under bias for storage • Charge is shifted out of a row or column by a shifting potential • Cannot be integrated on the same substrate as accompanying electronic circuits

8. CCD Operation

9. CCD Operation

10. CCD Operation • http://micro.magnet.fsu.edu/primer/java/photomicrography/ccd/shiftregister/index.html • http://www.extremetech.com/article2/0%2C3973%2C15465%2C00.asp

11. CMOS Cameras • Can be created with standard CMOS processes • Can be integrated with accompanying electronic circuits • An incident photon creates an electron-hole pair in a reverse biased diode • Configured to cause charge to drain off of a capacitor • Photon absorption  capacitor voltage decrease

12. CCD vs. CMOS Cameras • CCD has a better Fill Factor (FF) • Better image quality and photon capture • Lower noise • CCD only outputs the analog charge • Must be converted to digital by another chip • CMOS has on-chip integration • Results in high-speed and low-power • Reduces flexibility, but decreases cost

13. X-ray Imagers • Amorphous thin film techniques can produce large-area x-ray detectors • Two types: • Indirect • Direct • On-pixel amplification means fewer x-rays needed to make an image  Safer!

14. p-i-n Structure Ec h Ev i a-SiC:H n a-SiC:H Al p a-SiC:H ITO

15. X-ray Imagers • Indirect Method: • A top layer of phosphor turns the x-ray into a visible discharge • Visible photons are then detected by amorphous silicon (a-Si) p-i-n photodiodes

16. X-ray Imagers • Direct Method: • Amorphous selenium (a-Se) absorbs x-rays • A layer of a-Se with a huge E-field is used • It converts x-rays into electron-hole pairs • E-field separates them into current

17. IR Imagers • Two detection methods: • Quantum (photon  e-h pairs) • Thermal (photon  temp) • Useful in night vision systems • Police use them in Ontario to find pot grow houses

18. IR Imagers • Quantum Detection: • Photons have an energy hf = hc/ • If this energy is bigger than the bandgap of a detector material, e-h pairs will be created • IR has lower energies than visible, so the bandgap has to be reduced • Detector bandgaps can be tuned from 0eV up • These detectors must operate at very low temperatures • Restricted to special uses

19. IR Imagers • Thermal Detection: • IR photons will turn into heat when they hit certain materials • The heat can be detected and imaged • A pyroelectric material will generate a voltage or current proportional to the IR power shining on it

20. Microcalorimetric Sensors • A heated chamber is kept at a constant temperature • An incoming gas flow is burned • When the gas burns it releases heat energy • The released heat results in less heat from the chamber to keep a constant temperature • Released heat energy can be measure by how much less the chamber needs to heat the gas flow

21. Microcalorimetric Sensors

22. Electrochemical Cells • Use a catalyst to convert molecules to be measured into ions • Two modes of operation: • Amperometric: The ions are moved through a catalyst and electrolyte to create a current • Potentiometric: The ions charge a capacitor and appear as a voltage

23. Electrochemical Cells Amperometric Potentiometric

24. Acoustic Wave Devices • Tiny free-standing beams are created through micromachining • They have a mechanical resonance frequency () • They are coated in a polymer that adsorbs the specific molecules to be observed • More molecules stick  mass  

25. Acoustic Wave Devices

26. Gas-Sensitive FETs • A small channel lets gas pass between the gate and the substrate (channel) • The underside of the gate can be coated with a material to adsorb certain gasses • When the gasses adsorb into the coating, it changes the threshold voltage

27. Resistive Semiconductor Gas Sensors • O2 can act as a p-type dopant in silicon • It attacks point defects • The number of point defects increases with temperature • The Si must be heated • The more O2 in the silicon, the higher the conductivity

28. Resistive Touchscreens • Two flexible resistive layers are separated by a grid of spacers • When the two layers are pressed together the resistance can be measured between several points • This determines where the two resistive layers contacted

29. Resistive Touchscreens

30. Capacitive Touchscreens • A conductive layer is covered with a dielectric layer • The finger represents the other plate of the capacitor • A kHz signal is transmitted through the conductive plate, the dielectric, and the finger to ground • The current from each corner is measured to determine the touch location

31. Capacitive Touchscreens

32. Ultrasound Touchscreens • Ultrasonic sound waves (>40 kHz) are transmitted in both the horizontal and vertical directions • When a finger touches the screen, the waves are damped • Receivers on the other side detect where the sound was damped • Multiple touch locations are possible

33. Ultrasound Touchscreens

34. Fingerprint Sensors • An array of tiny capacitive sensors • Works similarly to the capacitive touchscreen • Finger works as one plate of a capacitor • Chip works as the other • Sensors are small enough to determine if a fingerprint ridge is touching it • An image is produced

35. Fingerprint Sensors

36. Thank You! • This presentation will be available on the web.